This invention relates generally to automatic gain control (AGC) circuits and, more particularly, to automatic gain control circuits containing both a temperature compensation and an exponential control function for the loop""s variable gain amplifier, and more specifically to the portion of the AGC circuit that provides the temperature compensation and the exponential transfer function.
Optimal operation and cost effective electronic systems can best be achieved by designing those amplifiers to operate on approximately constant amplitude peak-to-peak signal input envelopes. This is often accomplished by using an automatic gain control circuit (AGC).
Automatic gain control circuits (AGCs) are used in a wide variety of electronic devices to control the amplitude of input information waveforms. The output of the AGC is bounded within a prescribed range, allowing subsequent electronic amplifier circuits to operate on those waveforms within, and only within, their designed limits of linearity thereby preserving the totality of the information content of those input waveforms. Example applications include hard disk drive systems, communication systems, sensor systems with a varying input signal; an example sensor system might be an electronic glucose monitor. These examples are illustrative only, many other applications for signal conditioning circuits using AGC""s exist.
Without limiting the scope of this disclosure, in one application AGC""s are used in the data channel circuits for hard disk drive storage products. Hard disk drive digital magnetic recording channels typically present varying input signal envelopes to post processing electronic circuitry. This occurs because of drive-to-drive variations, head-to-head variations, sector-to-sector variations, and variations within a sector caused by changes in the magnetic properties of the storage media used in the disk drive. It is easier and more cost effective to design post processing circuitry which accepts fixed level or controlled level inputs than to design elaborate circuitry which will accept wide variations in input signals. In the case of hard disk drive read circuitry, it is an AGC circuit in the first stage of the read signal circuitry that removes the envelope variations of the input signal, while preserving the information content, thereby passing a fixed amplitude signal to subsequent circuitry. This fixed amplitude signal facilitates the design of simple, low cost, and efficient post-processing circuitry in the subsequent stages.
The basic form of an AGC loop, as shown in FIG. 1, consists of an alternating current (a.c.) coupled input (1) followed by a variable gain amplifier (VGA) (3) which drives a low pass filter (5) followed by an a.c. coupled output (7) to subsequent circuitry, with a feedback loop from the output (7) of the low pass filter through a peak detector (9) to an exponential voltage-to-current converter (11). Converter (11) that provides as output a control signal that controls the gain of the VGA (3).
In operation, the AGC feedback loop responds rapidly to the input signal because of the exponential characteristic of the transfer function within the voltage-to-current converter (11). This exponential characteristic equalizes the AGC performance. An ideal voltage to current converter circuit in an AGC loop provides a transfer function that is expressed as: output=ex, where x is a quantity proportional to an input signal. Usually the input will be a voltage from a peak detector circuit, but in other applications the input can take other forms. When the transfer function is ideal, the voltage-to-current converter provides a constant settling time for the feedback loop of the AGC for a variety of initial input signal conditions, which is very desirable. A well designed converter circuit for an AGC will provide a desired constant settling time independent of temperature and independent of process variations in the wafer process used to fabricate the circuitry.
In the prior art, exponential voltage-to-current converters have been designed exclusively for each wafer manufacturing process. These circuits have been complicated because of the need to provide temperature compensation. Without temperature compensation, the circuit performance will vary widely over a range of operating conditions, which results in unacceptable AGC performance.
Various approaches have been used to provide temperature compensation circuits for the exponential part of the AGC transfer function. Some prior art approaches provide for additional circuitry, which uses a PTAT (Proportional-To-Absolute-Temperature) current source, in the control path for driving the bases of a pair of differential transistors. The circuit is designed so that the temperature dependent terms in the numerator and denominator cancel each other out, making the entire circuit temperature independent for the prescribed ranges of operation. The gain function for the typical prior art AGC circuit is a hyperbolic function, which is approximately:             output      input        =                  e        x                    1        +                  e          x                      ;
with x being a value which is which is approximately exponential for the range within the boundaries of the xe2x88x92x, xe2x88x92y quadrant of the hyperbolic tangent function.
The gain is a hyperbolic transfer function, which approximates the desired exponential transfer function only for small values of the quantity x. Further, this gain transfer expression holds only if one of the current sources varies appropriately over temperature so that there is no temperature dependence. Thus the circuit requires a PTAT current source.
Although these prior art approaches can provide a converter circuit for an AGC that performs approximately like an ideal exponential circuit under certain conditions, neither of these patents provides a circuit for an AGC with an ideal transfer function. Further, prior art solutions often require a PTAT current source, or an offboard PTAT source from which the current can be derived.
A simple and efficient voltage to current converter circuit for use in AGC circuits, and other applications, is therefore desirable. The transfer function should be an exponential function that is temperature independent and process variation independent for good performance over a range of conditions.
In accordance with the principles of the present invention, there is disclosed herein an exponential voltage-to-current converter circuit. The circuit can be used in any application where an exponential transfer function is desired. When used within an AGC circuit a preferred embodiment of the circuit provides the necessary exponential transfer function independent of temperature and manufacturing process for the AGC control loop.
In accordance with a preferred embodiment of the present invention, the circuit is immune to process differences between manufacturing facilities and ambient temperature differences while providing the broadest linear range of VGA gain control possible for a given input signal, because of its ideal ex input-output characteristic.
The circuit of a preferred embodiment of the invention is a two stage circuit. A first differential amplifier is provided for receiving an input voltage and outputting a voltage, the differential amplifier optionally including internal feedback amplifiers for providing gain between the input terminals and the base terminals of the differential pair of transistors that make up the differential amplifier, the optional amplifiers providing improved temperature compensation. A second stage receives the output voltage and outputs a current that is related to the input voltage by a temperature independent exponential function which is proportional to the input voltage. The circuit includes a negative feedback loop for limiting the output current when the current through the feedback loop exceeds a predetermined limit.
The present invention provides significant benefits over the prior art, in that:
1) the circuit uses fewer devices in the implementation;
2) the transfer curve is a true exponential function as opposed to using one quadrant of a hyperbolic tangent as an approximation to an exponential, thereby providing more range of linearity and a broader input voltage range for the circuit over the prior art; and
3) the circuit uses the existing current sources of the device chip as opposed to PTAT current sources which require additional circuitry on or off-chip.